Electrical conductor or resistance and method of making the same



Aug. 14, 1956 D B, COX 2,759,041

ELECTRICAL CONDUCTOR OR RESISTANCE AND METHOD OF MAKING THE SAME Filed Sept. 5. 1952 INVENTOR um'du (0X BY KM md ATTORNEYS llnited States Patent ELECTRICAL CONDUCTOR R RESISTAMIECE AND 5 METHOD 0F MAKING THE SA Duncan B. Cox, Woodbury, N. Y. Application September 5, 1952, Serial No. 308,044

4 Claims. (Cl. 174-121) This invention relates to an improved electrical conductor or resistance of the type suitable for use as a heating element and to a method of making the same.

Attenuated conductors and resistance elements have relatively low tensile strength and this is particularly true where the conductor is made of a low fusing metal. An

object of the invention is to provide a device of the above character in which an improved tension or stress carrying insulating member is associated with the conductor or resistance vw're in such a manner that the tensile stresses are sustained directly by the stress carrying member,

rather than by the wire.

Another object is the provision of an improved method for making a conductor or resistance element having my stress carrying insulating member formed therearound.

A further object is the provision of an electrical conductor and resistance element having a stress carrying insulating member of the above character so constructed and arranged that tensile stresses applied thereto are transferred directly between the stress carrying member and any plug or electrical connector to which the conductor is attached.

This application is a continuation-in-part of my copending application Serial No. 88,949 tiled April 22, 1949, for Electric Heating Element, now Patent No. 2,610,286 granted September 9, 1952.

In the accompanying drawing:

Fig. l is a view of an improved conductor or resistance element embodying my invention with portions of the stress carrying member broken away to reveal the wire;

Fig. 2 is a similar view of the conductor or resistance element having an outer insulating casing or coating applied thereto;

Filg. 3 is a partially sectional and partially elevational view of the end portion of a heating tape incorporating my improved resistance elements and a plug arranged to insure direct transfer of the tensile stresses between the stress carrying casing and the plug;

Fig. 4 is an enlarged detailed view partially in section showing the connection between one of the resistance elements and the plug;

Fig. 5 is a perspective view of the terminals of the plug and the end portion of the heating tape prior to assembly; and

Fig. 6 is a cross-sectional view in the direction of the arrows on line 6-6 of Fig. 5 showing the construction of the heating tape.

My improved conductor and resistance element is shown most clearly at 10 in Figs. l and 2 and comprises generally an elongated attenuated conductor 11 of low tensile strength surrounded by a stress carrying insulat- 5 ing casing 12, the latter being usually covered by an additional insulating and protective coating or casing as shown at 13 in Fig. 2.

The wire 11 may be made of any metal or alloy having 70 the desired resistance in tine sizes, or which fuses at a relatively low temperature. For this purpose I prefer 5 than the adjacent areas.

to use copper, alloys of copper with nickel and chromiuml nickel resistance alloys, or Where low fusing is desired,

I may employ lead or zinc, or alloys of lead or zinc containing small percentages of one or more of the following i. e., copper, antimony, tin, bismuth or silver. Tin, cadmium or bismuth by themselves, or as the major constituents of an alloy can also be used in place of lead or zinc but these are not economically attractive at the present time.

The diameter of the conductor may vary in accordance with the desired resistance. I have found, however, that the practical lower limit is .0008 and the practical upper limit is .008 and the diameter should be conlned within these limits, except for the low fusing metals and alloys for which I have found the reasonable upper limit to be .020 above which fusing properties are not constant and the practical lower limit is .005". Where the resistance wire is to be used in a heating tape of the type shown in Figs. 3 to 6 inclusive, I have found that satisfactory results are obtained by making a conductor out of 4% antimonial lead with a diameter of .0119. This alloy melts at about 572 F. but in the form of extremely ne wire, and when covered by a suitable glass bre insulation it provides a high degree of safety against tire hazards in that it will readily fuse and open the circuit if inadvertently connected to too high a potential, or if a short circuit should develop due to any mechanical damage to the insulation. I have also found that if a heating cable, or any other appliance using my new resistance element is allowed to operate under conditions which restrict heat dissipation, excessive temperatures are prevented by the fusing of the conductor.

Resistance wire of the types and sizes used in my invention are extremely flexible. However, the stress which such wires can withstand in tension without stretching is extremely low because the metal is of relatively ine diameter. For example, the 4% antimonial lead wire for a diameter of .0119 will break with an applied load of less than 1/2 pound. Although the wire can stretch without breaking, any stretching tends to be concentrated locally and to reduce the diameter of the wire, thus causing a local increase in its resistance. The area affected will release more heat and be hotter in operation ecause of the relatively large temperature coeilicient of resistance of the wire, this increase in temperature further increases the resistance and the temperature. Therefore, wire which has been subject to local stretching is not suitable for use as a heating element.

I have found that it is possible to eliminate appreciable local stretching by applying to the Wire or conductor a tension member or stress-carrying insulating casing in the form of a special tubular braid 12 of glass fibre. When this braid is being applied the wire or conductor is subjected to only enough tension to cause it to unroll from its supply spool i. e., a force of l or 2 ounces but the braid is withdrawn from the braiding machine under relatively hi/gher tension so that all of the stretch in the libres which exists as a result of the braided construction is removed before the braid comes in contact with the wire. A special impregnation treatment is used to prevent the braid from loosening and contracting in length when the tension is removed.

As stated above, the tubular stress carrying braided casing is made from glass bres. Glass libre is particularly suited for this application due to its high strength and due to the fact that it has less stretch up to its breaking point than any other insulating fibre.

A preferred braiding construction uses a fibre glass yarn yielding 5000 yards per pound and consisting of 8 strands with 11 picks per inch. This braid is withdrawn 3 from the braiding machine at twice the speed usually used in applying braided insulation to wire. This higher speed reduces the angle between the strands and the wire from the normal angle of approximately 45 to approximately 26 or less which decreases the stretch of the finished braid up to its breaking point, and also increases its strength. For certain applications, I have found that 5 picks per inch is sufficient and gives improved strength and reduced finishing cost. Under these conditions the angle between the strands and the wire is correspondingly reduced below 26 to approximately 10. Although my braided covering gives poor coverage and is too loose for ordinary insulating use, after the special impregnation treatment it has a tensile strength of more than 30 pounds and provides the type of stress carrying covering which is needed to protect the weak wire or conductor.

As stated above, the braid must be subjected to a special impregnating treatment to prevent it from loosening up and contracting. Among the materials which I have found suitable for this impregnation are compounds of high melting waxes and watery emulsions of plasticized polyvinyl chloride. The tension on the braid must be maintained throughout the braiding operation and thereafter until after the impregnating compound has set, either by cooling as in the case of the hot melt, or by evaporation of the water, heat-curing and cooling, as in the case of the emulsion.

After the filler or impregnating compound has set, the glass bres are maintained by the ller substantially in the position corresponding to that when the casing was under tension, thereby preventing contraction of the casing. Thereafter, no special care is needed in handling the resistance element with its impregnated covering since the braid provides adequate protection for the wire.

For many applications it is desirable to provide a smooth outer surface to the tubular braid. This may be accomplished by providing the casing or coating 13, shown in Fig. 2, in the form of a coating of waterproof material having the required dielectric strength and abrasion resistance. I have found that a suitably compounded, high-molecular-weight vinyl chloride resin with a thickness of .010 makes a very satisfactory coating. This can readily be applied as a plastisol or by the extrusion process. An extruded coating of nylon .003 is also suitable for this purpose as are various other coatings applied in the usual manner from solvent solutions. For any of these coating materials, if the processing procedure involves temperatures or solvents which soften the impregnation for the braid so that the bres would tend to lose their taut position, it is necessary to maintain tension on the braid until after the impregnation has again set. This is necessary in order to preserve the non-stretchable, stress-carrying characteristics of the casing.

Tests have indicated that the fusing performance is improved by the rough inside surface of the braid. For some applications, a further improvement in fusing performance is obtained by applying a coating of flux, such as rosin, to the wire before applying the braid.

My improved conductor and resistance wire may be used for many different purposes, particularly as a heating` element for various devices and appliances, such as the heating tape shown in Figs. 3 to 6 inclusive. In using the conductor and resistance Wire the device should be assembled so that any applied tensile stresses are carried by the stress carrying braided casing and can not cause stretching of the wire itself. Since the wire is brought outside the casing only at the two ends of a given length for the purpose of making electrical connections, it is at these two points that special precautions must be taken to insure direct transfer of stresses between the stress carrying casing and the electrical connector.

In Figs. 3 to 6 I have shown a heating tape or ribbon of the type disclosed in my above referred to Patent No. 2,610,286, in the form of an elongated strip of ribbon 14 having longitudinal corrugations 15 interconnected by the flat web portions 16. The `body of the strip or ribbon is made of a plastic material having good dielectric properties. For this purpose, I prefer plasticized polyvinyl chloride. Embedded in the plastic body of the strip or ribbon are the conductors or resistance elements 10 having the glass braid insulating member applied thereto. Where the tubular glass braid is impregnated with plasticized polyvinyl chloride it can be locally bonded or heat sealed to the body of the ribbon by the application of dielectric heat at the end of the strip, after stripping the Wires bare for a short distance to make the desired electrical connections at the two ends. The body of the strip can then be mechanically fastened in the plug by the use of conventional solderless terminals or otherwise and will transfer stresses from the plug into the braided reinforcement without stressing the wire.

In the illustrated embodiment of the heating tape the stress carrying braid is impregnated with a wax and, therefore, cannot be made to adhere to the body of the strip 14. Under those circumstances the conductors may be connected to the terminals of a plug or electrical connector in the manner illustrated in Figs. 3 to 5 so as to insure direct transfer of stress between the plug and the stress carrying casing. Thus it will be seen that the two outer conductors or resistance elements are extended beyond the body of the plastic heating tape and, with the tubular casing 12 still applied thereto, knots 17 are formed therein and a flexible acetate cement is preferably applied to the knotted area of the glass braid casing. Provided the braid is not unraveled at the knotted area, such knots do not adversely affect the wire. The glass L casing preferably terminates just beyond the knotted area but the conducting wire 11 is extended therebeyond and is folded back upon the resistance element in a small loop as shown. The looped portion of the conductor and the entire resistance element immediately beneath the knot is then encased in the tubular sleeve 18 of a solderless terminal connector 19 which, in turn, is riveted to the prongs 20 of plug 21. Tubular sleeve 18 is tightly compressed around the resistance element to insure contact with the wire and prevent corrosion and so that if any attempt is made to withdraw it from the end of the element it will engage the knot 17. The neck portion of the terminal connector is preferably encased in an insulating sleeve 22 made of vinyl resin or the like.

The two adjacent or inner resistance elements of the heating tape are extended slightly beyond the body of the tape at the plug end thereof as shown in Figs. 3 and 5 and the glass braid casing is unraveled therefrom and the two wires are secured together as shown at 23. The connected wires and the unraveled glass casing are then embedded in polyamide resin which adheres to the fibres as shown at 24. The polyamide serves to protect the connected wires when they are inserted in the plug. Also, due to the adherence of the polyamide to the glass fibres it serves as an additional stop to prevent mitward movement of the plastic body of the tape.

If tensile forces are applied to the plug 21 they will be transmitted to the terminal connectors 19 which, in turn, will transmit them to the glass braid casing due to the interengagement of the necks of the terminal connectors with the knots 17. Thus, it will be seen that tensile stresses are not transferred to the conductors, but rather are transferred directly between the electrical connector and the glass braid casing. As shown in my above referred to Patent No. 2,610,286 the opposite end of the heating tape is also arranged so as to` prevent transfer of tensile stresses to the wires or conductors.

My improved conductor and resistance element 10 can be used for many other purposes than the heating tape 14 and can find application wherever it' is desired to have a heating element which is safe in operation and which can withstand reasonable tensile stresses.

It will be seen from the foregoing that I have provided an electrical conductor or resistance element ha ing an improved stress carrying insulating casing associated therewith. It will also be seen that I have provided an improved arrangement for transferring tensile stresses directly between an electrical connector and the stress carrying casing. In addition, I have provided an improved method for making electrical conductors or resistance elements embodying my improved stress carrying insulating casing.

Modifications may be made in the illustrated and described embodiment of my invention without departing from the invention as set forth in the accompanying claims.

I claim:

l. The method of forming an electrical conductor of the type having a relatively fragile wire with a stress carrying insulating casing disposed therearound which comprises: providing a supply of elongated attenuated wire; providing a supply of glass libre strands; withdrawing the wire from the supply under relatively low tension; withdrawing t'he glass iibre strands from their supply and forming them into a tubular braid around the Wire under relatively high tension as compared with the tension applied to the wire so as to cause the glass iibres to be disposed at an angle of no more than 26 to the longitudinal axis of the braid and so as to eliminate longitudinal stretch from the glass iibre braid; and providing a dielectric iller for the glass braid and causing said filler to set while maintaining the braid under relatively high tension as compared to the wire so as to prevent the contraction of the tubular braid.

2. An electrical conductor comprising an elongated, attenuated, fragile metal Wire and a stress carrying exible insulating casing non-stretchable in a longitudinal direction in the form of a flexible braided tube made of glass bres disposed at a relatively small angle in the order of not more than approximately 26 to the longitudinal axis of the tube and being impregnated with a dielectric iiller to prevent contraction thereof, said fibres being maintained by said iller substantially in the position corresponding to that when said casing is under tension, so as to prevent contraction of said casing when no tension is applied.

3. An electrical conductor as set forth in claim 2 in which the attenuated fragile metal wire has a diameter of no more than .020" and the flexible braided tube is provided with a smooth coating of dielectric material.

4. An electrical conductor comprising an elongated, attenuated, fragile metal Wire, a stress carrying liexible insulating casing non-stretchable in a longitudinal direction in the form of a liexible braided tube made of glass fibres disposed at a relatively small angle in the order of not more than approximately 26 to the longitudinal axis of the tube and being impregnated with a dielectric iller and being maintained by said liller substantially in the position corresponding to that when said casing is under tension to thereby prevent contraction of said casing when no tension is applied, and a terminal for said conductor provided with a sleeve-like portion, said wire and stress carrying casing being knotted adjacent one end and being disposed in the sleeve-like opening with the knot engageable therewith so as to transfer stresses from said casing to the terminal, a portion of the wire beyond the knot being free of such stress carrying casing and being in electrical contact with the terminal.

References Cited in the file of this patent UNITED STATES PATENTS 306,478 Gardner Oct. 14, 1884 893,873 Ritter July 21, 1908 2,230,481 Bromley Feb. 4, 1941 2,234,560 Keyes Mar. 11, 1941 2,243,560 Hall et al May 27, 1941 2,260,024 Hall et al Oct. 21, 1941 2,594,838 Alexander et al. Apr. 29, 1952 2,610,286 Cox Sept. 9, 1952 FOREIGN PATENTS 520,246 Great Britain Apr. 18, 1940 

